This application pertains to the art of manufacturing plastic tubing and more particularly to the art of manufacturing semiround plastic tubing, conduit, or pipe. The invention is particularly applicable to corrugated plastic drainage tubing and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications, such as manufacturing smooth-walled semiround tubing, expanded plastic semiround tubing, septic and leachbed tubings, subterranean conduit, other fluid conveying channels, pipes for shielding electrical conduit, and the like.
In the past, most corrugated drainage tubing has been circular in cross section. The circular tubing is generally manufactured in a linear production line. A plastic extruding machine fitted with a circular extrusion die receives pellets of polymeric, thermoplastic material and extrudes a plastic sleeve through an extrusion die. The extruder subjects the pellets to a combination of heat and pressure to soften the pellets to a semifluid, plastic state. Closely adjacent the extruder is positioned a continuous blow molding assembly. Conventionally, the blow molding assembly includes pairs of mold blocks arranged in continuous loops. Adjacent the extruding die, each pair of mold blocks abuts together to define the circular corrugated mold. The mold blocks then travel linearly away from the die as the sleeve is forced by pneumatic pressure to conform to the shape of the interior surface of the mold blocks. While the mold blocks of each pair are abutting each other and traveling in contact with the tubing, the plastic material cools sufficiently that it retains its molded shape. The mold blocks then separate and return to their initial position.
After the tubing is molded, it is cooled in a water bath to make the plastic sufficiently hard for slotting or drilling operations. If the tubing is to be used for drainage tubing, the continuous length of tubing is commonly conveyed through a slotting machine which cuts a series of thin slots into the side walls of the tubing. If the tubing is to be used for septic tubing, the tubing is commonly conveyed through a drilling machine which drills round holes about 5/8 of an inch in diameter into the side walls of the tubing. If the tubing is to be used for conveying fluids only, it is neither slotted nor drilled. The plastic tubing is commonly cut at 10 foot lengths or coiled in rolls, such as a 250 foot roll. Numerous patents illustrate the conventional methods for manufacture of corrugated plastic tubing, see for example U.S. Pat. No. 3,732,046, issued May 8, 1973 to R. C. Martin; U.S. Pat. No. 3,819,778, issued June 25, 1974 to E. J. Maroschak; U.S. Pat. No. 3,843,758, issued Oct. 22, 1974 to E. J. Maroschak; and U.S. Pat. No. 3,990,827, issued Nov. 9, 1976 to E. J. Maroschak.
Others have manufactured smooth-walled circular tubing for drainage, septic, and other applications. The smooth-walled tubing has been manufactured of a single plastic material by some and by others with laminated plastic materials. Expanded plastic has been used for the single plastic material and for one or more layers of the laminated tubing. To form laminated tubing, the extruding machine includes an extruder for each plastic material and an extrusion die with concentric outlets. An extruder plasticises each type of plastic material in the laminant. The tooling of the extruder includes channeling means for channeling each type of plastic to one of the concentric outlets of the die. The smooth-walled tubing is similarly cooled and cut in 10 foot lengths. The lengths are slotted or drilled, if their intended application so requires.
The pellets of polymeric material fed to the extruding machine are commonly a blend of polymers which have different properties. A polymer with an advantageous property normally has offsetting disadvantageous properties. Further scrap plastic from drillings, trim, out-of-spec. production, and the like are commonly reprocessed and mixed with virgin plastic pellets. The resultant plastic product has an average of all the advantageous and disadvantageous physical properties. Some of the polymers are chosen for a high structural strength, others for a high impact resistance, still others for a high abrasion resistance, and the like. The blended product will not be as structurally strong as it would if it were composed entirely of the structurally strongest component, nor as abrasion resistant as it would have been if it were composed entirely of the most abrasion resistant component, and the like.
One of the problems with the prior art apparatus and methods for manufacturing corrugated plastic tubing is that only a single length of tubing is manufactured on each production line.
Another problem with the prior art manufacturing techniques is the difficulty encountered when drilling or slotting moving lengths of continuous plastic tubing.
Yet another problem with the prior art manufacturing techniques is the difficulty encountered in trying to optimize the physical properties of the plastic material. Blending a plurality of plastic materials fails to achieve the most advantageous physical properties of any one of the individual components.
The present invention contemplates a new and improved method and apparatus for manufacturing plastic tubing which overcomes the above-referenced problems and others. It provides an apparatus and method for manufacturing plastic tubing which is simpler, more economical and faster than the prior art.